The present invention relates to a method for stabilizing the intensity of a light beam which is to be deflected by acoustooptical deflector elements, without regard to deflection angles, and more particularly relates to a method for stabilizing the intensity of the light beam which is emitted by a laser, and which is to be deflected by the acoustooptical deflector elements arranged in a scanning exposure head of a picture reproducing machine such as a color scanner, a color facsimile, or the like, without regard to the deflection angles.
In an exposure head for a picture scanning reproducer, and the like, a laser tube is often used nowadays, because of its high brightness, monochromaticity, parallellism of beam, and so forth. An acoustooptical deflector element has been used for deflecting the laser light beam which is passed therethrough. Thus, an exposure head comprising a combination of a laser tube and an acoustooptical deflector element has been developed.
However, in such an exposure head, the intensity of the laser beam from the laser tube varies over a relatively long time period, and the intensity of the light beam deflected by the acoustooptical deflector element also varies depending on the deflection angles. In order to stabilize the intensity of the laser beam various methods have been proposed.
In one conventional stabilizing method, a part of the light beam from the laser tube is reflected by a beam splitter, and the intensity change of the light beam is detected by a light detector. Depending on the intensity change detected, a correction signal in reverse phase to the intensity change of the light beam is produced. The correction signal is amplitude-modulated upon a high-frequency carrier signal. The light beam is then modulated in an optical modulator element by the amplitude-modulated signal, thereby stabilizing the intensity of the light beam.
This method is very efficient for a light beam which varies over a relatively long time period. In some applications, the light beam is deflected in an extremely short time, such as a few microseconds by using an acoustooptical deflector element. For example, in the production of halftone dots in a picture reproducing machine such as a color scanner, the feedback of the correction signal for correcting the intensity change of the light beam, depending on the deflection angles is delayed, and accordingly the response of the acoustooptical deflector element is delayed, with the result that a negative feedback often becomes a positive feedback due to the phase delay of the feedback signal. Hence, in this method, the feedback control can hardly be utilized.
In order to avoid this defect, an electrooptical modulator is used instead of the acoustooptical deflector element. However, the electrooptical modulator is very unstable in presence of the temperature changes, and it is difficult to maintain the temperature of the electrooptical modulator, to say nothing of the high cost of such devices.
Alternatively, when the light beam is stabilized by feedback control by using the acoustooptical deflector element, in order to reduce the time delay, the light beam is passed very near an ultrasonic wave signal generator, accordingly, the light beam is affected by the influence of the heat generated by the ultrasonic wave signal generator.